1. Field of the Invention
The invention relates generally to electrochemical capacitor cells, fuel cells, and energy harvesting.
2. Description of the Prior Art
The power source of choice for autonomous electronic devices is batteries, but they must be either continuously replaced or recharged when they run out of energy. Similarly, fuel cells and other fuel conversion systems require fuel replenishment. For some autonomous devices, a maintenance free situation is highly desirable, so that the device can operate unattended for days, months, or years. The power source for the device often must have the ability to operate at different power levels, for instance at low power while collecting data, and at high power during data transmission.
Energy harvesting devices provide a means to recharge batteries or supply energy directly to a device in a maintenance-free situation. For instance, solar cells can be used to charge batteries during the day so that the energy can be used at night. Another energy harvesting device includes elastomeric polymers that can be used on heel strikes to harvest the energy that a person generates while walking. A disadvantage of energy harvesting methods is that they can usually only be used in specific conditions (e.g. sunlight or under compression). They also generally have low power (on the order of microwatts per square centimeter) and can be used to operate only low-power devices.
Another energy harvesting option is to power a device by scavenging the hydrogen available in the atmosphere. Hydrogen is the best fuel for fuel cells in which hydrogen and oxygen gas react at different electrodes to create electricity and water. Although the hydrogen is available in the ambient at ppm levels, it is always available. In a fuel cell, hydrogen oxidation occurs at the anode or negative electrode of the cell and oxygen reduction occurs at the cathode or positive electrode. In a device that utilizes the hydrogen from the environment, the anode or negative electrode should be catalytically active for hydrogen oxidation and the cathode should be catalytically active for oxygen reduction.
Hydrous ruthenium dioxide (which is designated in the literature as RuOxHy, RuO2.xH2O, and hydrous RuO2) is a charge-storage material in ultracapacitors. It has a maximum specific capacitance of 700 to 800 F/g when it has been heated at 150° C. and has a composition of approximately RuOx.0.5H2O. This maximum in capacitance has been ascribed to the point at which the competing protonic and metallic transport mechanisms in the hydrous RuO2 are both optimized. The mixed protonic and metallic conductivity of hydrous RuO2 plus its electrocatalytic nature also makes it useful as an electrocatalyst for brine oxidation in dimensionally stable anodes, methanol oxidation at Pt—Ru anodes, and water oxidation with Ce4+. Ruthenium oxides are also active for oxygen reduction.
Aqueous RuO2 ultracapacitors typically comprise symmetric hydrous RuO2 positive and negative electrodes and a 5 M sulfuric acid (H2SO4) electrolyte. The uncharged symmetric electrodes have an open circuit voltage (OCV) of 0 V, but the electrodes can hold potential differences up to 1.4 V after charging. The accepted mechanism for charge storage in hydrous RuO2 is via the “double-insertion” of electrons and protons into the structure. As the positive electrode is discharged, the average oxidation state of the Ru is reduced from 4+, to 3+ and then 2+. The opposite trend occurs at the negative electrode. Because hydrous RuO2 is predominantly metallic, most of the electronic states are delocalized, and the oxidation states are averaged. The discharge and charge reactions are given in formulas 1 and 2, respectively.
Discharge Mechanism of Positive Electrode:RuOx(OH)y+δH++δe−→RuOx−δ(OH)y+δEmax=1.4V vs.NHE  (1)Charge Mechanism of Negative Electrode:RuOx−δ(OH)y+δ→RuOx(OH)y+δH++δe−Emin=˜0V  (2)
A major drawback of RuO2 ultracapacitors is their tendency of the electrodes to undergo self-discharge and potential recovery resulting in a decrease in cell voltage (and loss of power) over time. Self-discharge refers to the decrease in voltage that occurs after the positive electrode has been charged. Potential recovery describes a minor reaction at the negative electrode—after charging, the voltage of the negative electrode drifts positive.